A gas diffusion electrode substrate installed in a fuel cell has conventionally been a porous electrode substrate composed of a paper-like carbon/carbon composite which was obtained by forming short carbon fibers into a paper, then binding the short carbon fibers to each other via an organic polymer, and firing the paper at high temperature to carbonize the organic polymer, in order to increase mechanical strength (see Patent Literature 1). In addition, for the purpose of achieving lower cost, a porous electrode substrate which was obtained by forming oxidized short fibers into a paper, and firing the paper at high temperature to carbonize the oxidized short fibers is proposed (see Patent Literature 2). Further, a proposal has been made (see Patent Literature 3) for mat that includes a plurality of carbon fibers; and a gas diffusion layer for a fuel cell that is obtained by incorporating a plurality of acrylic pulp fibers into the carbon fiber mat and then by curing and carbonizing them.